A motor can be combined with a compressor in a single housing to provide a motor-compressor system. Using a shared shaft, or two or more shafts coupled together, the motor drives the compressor in order to generate a flow of compressed process gas. In hermetically-sealed motor-compressors, the shaft is typically supported by two or more radial magnetic journal bearings and often includes at least one axial magnetic bearing for thrust compensation. The magnetic bearings may be passive magnetic bearing systems using permanent magnets, or they may include active magnetic bearing systems having one or more electromagnets actively controlled by an external power source adapted to centralize or otherwise levitate the shaft.
Magnetic bearings installed within a hermetically-sealed motor-compressor are typically pressurized to a level close to the process inlet pressure, and cooled by process gas derived from the compressor and circulated via a cooling loop. Although the cooling process gas is ordinarily first treated in a gas conditioning skid to remove contaminants and free liquids, there is still potential for the generation and accumulation of liquids within the cooling loop. For example, liquids such as water, hydrocarbon condensate, or other wellstream fluids can often form, and magnetic bearings are particularly susceptible to damage if they come into contact with these liquids or “dirty” cooling process gas. In such cases, the resistance to ground of the electrical windings of the bearings may be reduced which, if not reversed or at least stopped, could eventually lead to the complete failure of the bearing.
One way to protect the electrical windings from liquid penetration is the application of vacuum-pressure impregnation (VPI) to the windings which provides a protective coating that insulates the windings. Magnetic bearings, however, are subjected to repeated pressurization-depressurization cycles which increase the risk of liquids penetrating the VPI coating over time. Once the VPI coating is penetrated, the bearing coil resistance to ground gradually diminishes, and if the liquid penetration is not reversed or stopped, the coil resistance to ground will eventually become zero, thereby causing the bearing to short out and fail.
What is needed, therefore, is a method and system for monitoring the bearing coil resistance to ground as an indicator of the accumulation of liquids in the cooling loop of a motor-compressor, and more particularly within the magnetic bearings.